Recently, from the viewpoint of environment and fuel economy, hybrid electric vehicles (HEV), electric vehicles (EV) and fuel cell vehicles have been manufactured and commercialized, and undergoing further developments. These electrically powered vehicles absolutely require the utilization of a dischargeable and chargeable power supply. As the power supply, there are utilized a secondary battery e.g. a lithium-ion battery and a nickel-hydrogen battery, an electric double-layer capacitor and the like. Particularly, the lithium-ion battery is considered to be suitable for electrically powered vehicles because of its high energy density and high durability to repeated charge and discharge, and therefore developments thereof are variously and eagerly being made. However, when applying the lithium-ion battery to a power supply for powering motors of the above-mentioned various kinds of automotive vehicles, it is necessary to use two or more secondary batteries electrically connected in series in order to ensure a sufficiently large output power.
However, when batteries are connected through an external electrical connection, the output power is decreased due to the electrical resistance of the electrical connection. Furthermore, batteries having an electrical connection are spatially disadvantageous. In other words, due to the presence of the electrical connection, decreases in output power density and energy density of the batteries are brought about.
For the purpose of solving the above problems, there has been developed a bipolar lithium-ion secondary battery such as a bipolar lithium-ion secondary battery. The lithium-ion secondary battery is provided to have a power generation element formed by laminating a bipolar electrode two or more times through an electrolyte layer and a separator, the bipolar electrode being composed of a collector having on its one surface a positive electrode active material layer and on its another surface a negative electrode active material layer.
In order to ensure a greater output power density, the collector thus used in the bipolar lithium-ion secondary battery is preferably formed of a material which is lightweight and excellent in electrical conductivity. In view of this, it has recently been proposed to use as a material for a collector (a resinous collector) a polymer material to which an electrically conductive material is added. For example, in Japanese Patent Application Publication No. 2006-190649, a resinous collector obtained by mixing a polymer material with an electrically conductive material such as metal particles and carbon particles is disclosed.
However, the resinous collector as disclosed in Japanese Patent Application Publication No. 2006-190649 is high in resistivity in the surface direction and therefore the variability in electric current distribution is caused in the surface of the active material layer (at the time of charge and discharge) to locally make an overcharged region, so that there are some cases where the deterioration of the electrode is accelerated thereby.
In view of the above, an object of the present invention is to provide a bipolar electrode the deterioration of which can be suppressed.